Vanadium/phosphorus mixed oxide catalyst precursor

ABSTRACT

A process for the preparation of a vanadium/phosphorus mixed oxide catalyst precursor is described, comprising the reaction of a vanadium source in selected organic media in the presence of a phophorus source. The medium comprises: (a) isobutyl alcohol or a mixture of isobutyl alcohol and benzyl alcohol and (b) a polyol in the weight ratio (a) to (b) of 99:1 to 5:95. After its activation, the vanadium/phosphorus mixed oxide catalyst precursor is an excellent catalyst in the conversion of non-aromatic hydrocarbons like n-butane to malic anhydride.

This is a 371 of International Patent Application PCT/EP00/04939, filedon May 5, 2000, which has priority benefit on Italian Patent ApplicationMI99A001233, filed on Jun. 1, 1999.

The invention relates to a process for the production of avanadium/phosphorus mixed oxide catalyst precursor, its transformationinto the active catalyst and a process for the production of maleicanhydride using this catalyst

Maleic anhydride is a well known and versatile intermediate formanufacturing unsaturated polyester resins, pharmaceuticals oragrochemicals. It is usually produced by catalytic partial oxidation ofaromatic (e. g., benzene) or non-aromatic (e. g., n-butane)hydrocarbons.

The main component of the active catalyst in the oxidation ofnon-aromatic hydrocarbons like n-butane to maleic anhydride is vanadylpyrophosphate, (VO)₂P₂O₇, which as a rule is obtained by thermaltreatment of vanadyl acid orthophosphate hemihydrate of the formula(VO)HPO₄•0.5H₂O, acting as catalyst precursor. Both vanadylpyrophosphate and vanadyl acid orthophosphate hemihydrate may, ifdesired, be accompanied by a promoter element selected from the groupsIA, IB, IIA, IIB, IIIA, IIIIB, IVA, IVB, VA, VB, VIA, VIB and VIIIA ofthe periodic table of elements, or mixtures of such elements.

Methods for preparing the precursor compound conventionally involvereducing a pentavalent vanadium compound under conditions which willprovide vanadium in a tetravalent state (average oxidation number +IV).

Prior art knows a great many different procedures, which however ingeneral involve the use of vanadium pentoxide (V₂O₅) as a source ofpentavalent vanadium and orthophosphoric acid (H₃PO₄) as the phosphorussource (see e.g. U.S. Pat. No. 5,137,860 or EP-A-0 804 963).

As a reducing agent in principle any inorganic or organic compoundcontaining elements which are able to act as a redox couple possessingan oxidation potential suitable for this kind of reaction may besuitably applied.

The most common reducing agent is hydrogen chloride in aqueous solution.

Also favourably applied are organic media like primary or secondaryaliphatic alcohols or aromatic alcohols such as benzyl alcohol as thesecompounds seem to at least in part dissolve the reactants and therebyfacilitate the redox reaction.

The most preferred organic reducing agent is isobutyl alcohol asisobutyl alcohol combines optimal characteristics such as (i) a boilingpoint of 108° C. at atmospheric pressure, (ii) dissolution of thevanadium alcoholates formed from V₂O₅, thus favouring a complete redoxreaction in the liquid phase and (iii) achieving a redox potential forthe couples isobutyl alcohol/isobutyraldehyde and isobutylalcoholrisobutyric acid suitable to let the alcohol act as reducingagent. The tetravalent vanadium reacts with phosphoric acid (H₃PO₄) andleads to precipitation of the precursor vanadyl acid orthophosphatehernihydrate of the formula (VO)HPO₄•0.5H₂O. The precipitate is usuallywashed with isobutyl alcohol and subsequently dried.

A major disadvantage of the conventional method as described above isthat even after drying the precursor contains some percent of organiccompounds from the organic reaction medium, compounds which aresupposedly either (i) strongly adsorbed at the solid surface, andtherefore not easily removable by the washing and drying treatment, or(ii) physically trapped in between the crystals of the precursor, or(iii) physically or chemically trapped a (“intercalated”) in thecrystalline structure of the precursor.

It has been found that this percentage of organic compound which remainstrapped in the precursor is a fundamental parameter which can adverselyaffect the performance characteristics of the active catalyst obtainedafter the thermal treatment.

The object of the present invention therefore was to provide a methodfor controlling the carbon content in a vanadium/phosphorus mixed oxidecatalyst precursor and accordingly to provide a superior catalystprecursor which, when activated, leads to superior results in theconversion of a non-aromatic hydrocarbon to maleic anhydride.

It was found that the objectives could be achieved with a new processfor the preparation of a vanadium/phosphorus mixed oxide catalystprecursor according to claim 1.

The invention comprises reducing a source of vanadium in the presence ofa phosphorus source in an organic medium which comprises

(a) isobutyl alcohol or a mixture of isobutyl alcohol and benzyl alcoholand

(b) a polyol

in the weight ratio of 99:1 to 5:95.

In a mixture of isobutyl alcohol and benzyl alcohol the benzyl alcoholcontent is as a rule between 5 and 50 wt %.

As a source of vanadium a tetravalent or pentavalent vanadium compoundmay be applied. Representative examples, although not limiting, arevanadium tetrachloride (VCl₄), vanadium oxytribromide (VOBr₃), vanadiumpentoxide (V₂O₃), vanadyl phosphate (VOPO₄•nH₂O) and vanadium tetraoxide(V₂O₄). Vanadium pentoxide is the preferred vanadium source.

As mentioned above, the vanadium source may, if desired, be accompaniedby promoter elements selected from the groups IA, IB, IIA, IIB, IIIA,IIIIB, IVA, IVB, VA, VB, VIA, VIB and VIIIA of the periodic table ofelements, or mixtures thereof.

Preferred promoter elements are selected from the group consisting ofzirconium, bismuth, lithium, molybdenum, boron, zinc, titanium, iron andnickel.

Orthophosphoric acid (H₃PO₄) is the preferred phosphorus source.

Isobutyl alcohol is the preferred component (a).

Polyols which can be used as the component (b) are expediently C₂₋₆aliphatic polyols, preferably C₂₋₆-alkanediols such as 1,2-ethanediol,1,2-propanediol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol,1,4-butanediol, 2,3-butanediol, 1,2-pentanediol, 1,3-pentanediol,1,4-pentanediol, 1,5-pentanediol, 2,3-pentanediol, 2,4-pentanediol,1,2-hexanediol, 1,3-hexanediol, 1,4-hexanediol, 1,5-hexanediol,1,6-hexanediol, 2,3-hexanediol, 2,4-hexanediol, 2,5-hexanediol and3,4-hexanediol.

Most preferred polyols are the C₂₋₄-alkanediols 1,2-ethanediol,1,3-propanediol and 1,4-butanediol.

The preferred mixture of alcohols contains 5 to 30 mol % of polyol withrespect to isobutyl alcohol.

As a rule the vanadium source together with the phosphorus source issuspended in the organic medium and the mixture is kept under agitationat a temperature of expediently 90° C. to 200° C., preferably 100° C. to150° C. over a period of 1 h to 24 h.

The ratio of vanadium source to phosphorus source is conveniently suchthat the P/V atomic ratio is in the range of 1:1 to 1.3:1, preferably1.1:1 to 1.2:1.

As a rule after the reduction the precursor vanadyl acid orthophosphatehemihydrate of the formula (VO)HPO₄.0.5H₂O is formed which is filtered,washed and subsequently dried at a temperature of expediently 120° C. to200° C.

Due to the reduction treatment according to the invention the carboncontent of the precursor can be controlled in the range of 0.7 wt. % to15.0 wt. %, preferably in the range of 0.7 wt. % to 4 wt. %.

It has been found that best results are obtained with catalystprecursors which, after an additional thermal treatment at about 300° C.for about 3 hours in air have a residual carbon content of 0.7 wt. % to3 wt. %, most preferably 0.8 wt. % to 1.5 wt. %.

Once prepared the precursor can in view of its further activationtreatment be formed into defined structures with defined properties.Such procedures may include wet grinding to a specific particle size,the addition of additives to improve attrition resistance, and the aformation of a convenient shape.

A spherical shape for instance is most suitable for the application ofthe catalyst in a fluidized bed.

The further transformation of the so formed precursor into the activecatalyst can be performed following a great number of activationprocesses known in the art, but in general include a heat treatmentapplying temperatures of up to 600° C. More in detail, these processesmay involve:

(a) an initial heating of the precursor to a temperature not to exceed250° C.

(b) a further heat treatment from about 200° C. to at least 380° C. to600° C. at the maximum

(c) maintaining the temperature of stage (b) over a certain time and

(d) cooling the activated catalyst, thereby maintaining an individuallycontrolled atmosphere in all steps.

In a preferred embodiment, the activation of the precursor isaccomplished using the procedure described in EP-A-0 804 963.

In a more preferred embodiment, the activation comprises the steps of

(a) heating the catalyst precursor from room temperature to aprecalcination temperature of about 300° C. in air or oxygen-depletedair

(b) keeping at said pracalcination temperature,

(c) further heating the precalcined catalyst precursor in nitrogen up toa calcination temperature of about 550° C. and

(d) keeping at said calcination temperature.

After the transformation into the active catalyst the catalyst is readyto be applied for the conversion of non-aromatic hydrocarbons to maleicanhydride.

Such processes are well known in the art, e. g. from U.S. Pat. No.4,594,433, U.S. Pat. No. 5,137,860 or U.S. Pat. No. 4,668,652.

In general the non-aromatic hydrocarbon is converted with oxygen or anoxygen containing gas at a temperature from about 320° C. to 500° C. tomaleic anhydride. The non-aromatic hydrocarbon is expediently selectedfrom aliphatic C₄₋₁₀ hydrocarbons, preferably nbutane. The conversioncan take place in a fixed bed or a fluidized bed reactor but preferablyin a fluidized bed reactor. The following examples are given by way ofillustration only and arc not construed as to in any way limit theinvention.

EXAMPLES

In the following examples, the carbon content was determined bycombustion in pure oxygen at high temperature using the apparatus andprocedure described below and detection of the carbon dioxide formed byinfrared analysis.

Apparatus: ELTRA 900CS Measuring range: 0.001-100 wt. % C Sensitivity:0.0001 wt. % C Time per sample: 90 s Sample size: 0.1-0.5 g Oventemperature: 400-1500° C. Oxygen purity: 99.5% min. Oxygen flow rate: 4l/min

Procedure:

The furnace was heated up to 1330° C. and oxygen flow was opened 10minutes before starting the analysis. High carbon content detector wasselected and calibrated with standard samples having known carboncontent The sample size used was 150±10 mg.

Comparative Example 1 (Following Example 1 (Comparison) of EP-A 0 804963)

Into a three necked 1—1 flask fitted with thermometer, mechanicalstirrer and packed glass distillation column with reflux condenser andwater separator (cf. example 5), were introduced 8.20 g of V₂O₅ and 10.1g of H₃PO₄ (100%), suspended in 75 ml of isobutyl alcohol (99%+). Themixture was then kept under agitation and heated up to reflux and leftat these conditions for 6 h. The colour of the mixture changed fromred-brown to dark green and then finally to bright blue.

The mixture was cooled to room temperature, then filtered and washedwith a large excess of isobutyl alcohol. The solid was then dried in airat 125° C. for 5 h. The carbon content of the dried precipitate was 0.6wt. %.

The solid was then treated in air by heating from room temperature to300° C. (heating rate 1 K/min), then left at 300° C. for 6 h, heated inN₂ up to 550° C. (heating rate 1 K/min) and finally left at 550° C. for6 h. After the precalcination step (in air at 300° C.), the residualcarbon content was 0.5 wt. %.

Comparative Example 2

The preparation of the precursor was done as described in comparativeexample 1, but the thermal treatment was done according to the procedureof example 4 of EP-A-0 804 963:

(a) heating in air from 25° C. to 180° C. at a heating rate of 4 K/min

(b) further heating from 180° C. to 425° C. in a mixture of air (70%vol) and steam (30% vol) at a heating rate of 1.5 K/min

(c) isothermal step at 425° C. in the same atmosphere as instep (b), for2 h

(d) isothermal step at 425° C. in an atmosphere of nitrogen (70% vol)and steam (30% vol) for 3 h

(e) cooling in a mixture of nitrogen and steam at a rate of −2 K/min.

Comparative Example 3

The preparation of the precursor was done as described in comparativeexample 1, but the isobutyl alcohol was replaced by 35.5 g of1,3-propanediol.

The dried catalyst precursor had a carbon content of 11.6 wt. %.

The activation was performed according to the procedure of comparativeexample 1.

Example 1

4.11 g of V₂O₅ and 5.11 g of H₃PO₄ (100%) were suspended in 37.5 ml of amixture of 1,2-ethanediol/isobutyl alcohol (20/80 v/v). The mixture waskept under agitation and heated up to reflux, and left at theseconditions for 6 h. The color of the mixture changed from red-brown todark green and then finally to bright blue.

The mixture was cooled to room temperature, then filtered and washedwith a large excess of isobutyl alcohol. The solid was then dried in airat 125° C. for 5 h.

The dried catalyst precursor had a carbon content of 2.3 wt. %, whileafter the precalcination treatment in air at 300° C. the amount ofresidual carbon was 1.2 wt. %.

The activation was performed according to comparative example 1.

Comparative Example 4

The same procedure as in example 1 was carried out, with the exceptionof 1,2-ethanediol being replaced with benzyl alcohol.

The dried catalyst precursor has a carbon content of 1.6 wt. % whileafter precalcination the residual amount of carbon was 0.4 wt. %.

The activation was performed according to the procedure of comparativeexample 1.

Example 2

8.20 g of V₂O₅ and 10.07 g of H₃PO₄ (100%) were suspended in 75 ml of amixture of 1,3-propanediol/isobutyl alcohol (20/80 v/v). The mixture waskept under agitation and heated up to reflux, and left at theseconditions for 6 h. The color of the mixture changed from red-brown todark green and then finally to bright blue.

The mixture was cooled to room temperature, then filtered and washedwith a large excess of isobutyl alcohol. The solid was then dried in airat 125° C. for 5 h.

The dried catalyst precursor had a carbon content of 2.8 wt. % whileafter precalcination the residual amount of carbon was 1.8 wt. %.

The activation was performed according to the procedure of comparativeexample 1.

Example 3

The procedure of example 1 was repeated, but as reducing agent 37.5 mlof a mixture of 1,4-butanediol/isobutyl alcohol (20/80 v/v) was chosen.

The dried catalyst precursor had a carbon content of 1.6 wt. % whileafter precalcination the residual amount of carbon was 1.1 wt %.

The activation was performed according to the procedure of comparativeexample 1.

Example 4

The procedure of example 1 was repeated, but as reducing agent 37.5 mlof a mixture of 1,3-butanediol/isobutyl alcohol (20/80 v/v) was chosen.

The dried catalyst precursor had a carbon content of 1.6 wt. % whileafter precalcination the residual amount was 1.4 wt %.

The activation was performed according to comparative example 1.

Example 5

The procedure of example 3 was repeated (using the apparatus describedin comparative example 1), but the water generated during the reactionwas partially removed by azeotropic distillation.

The dried catalyst precursor had a carbon content of 2.3 wt. % whileafter precalcination the residual amount was 1.5 wt %.

The activation was performed according to comparative example 2.

Catalytic Tests

The catalytic tests were performed at atmospheric pressure in afixed-bed stainless steel laboratory reactor (length 25.4 cm, diameter1.27 cm) loaded with 3 g of catalyst. The products were collected andabsorbed in anhydrous acetone and analysed by gas chromatography. Theperformance of the catalyst was determined on the basis of the percentconversion of n-butane fed to the reactor (together with oxygen andhelium), the yield of maleic anhydride recovered in the absorber (MAyield) in % and the selectivity of the conversion towards maleicanhydride (MA selectivity) in %.

The following conditions were maintained during the tests:

W/F (weight of catalyst/total volumetric flow rate): 1.3 g.s/ml

feed composition: 1.7% n-butane, 17.2% O₂, rest He

measurement: after 200-300 h time-on-stream (stable catalyticperformance).

The results obtained for the various catalysts are summarised in table1.

TABLE 1 Example No. [° C.] Conversion [%] MA Yield [%] MA Selectivity[%] Comp. 1 360 9.8 6.4 65.3 380 16.7 11.4 68.3 400 26.3 17.2 65.4 42039.5 26.1 66.1 440 53.8 32.9 61.2 Comp. 2 360 12.2 8.7 71.3 380 18.012.4 68.9 400 26.8 18.2 67.9 420 39.6 25.5 64.4 440 56.0 34.4 61.4 46071.5 40.3 56.4 Comp. 3 360 9.5 0.6 6.3 380 30.5 2.1 6.9 400 40.6 2.3 5.7420 43.6 3.6 8.3 Comp. 4 360 11.1 7.5 68 380 17.1 11.7 68.5 400 26.517.5 66.2 420 39.5 25.7 65 440 55.1 35.0 63.5 1 360 16.7 12.1 72.2 38034.9 24.0 68.7 400 42.3 29.7 70.2 420 47.2 30.4 64.5 2 360 20.6 15.374.3 400 47.6 33.9 71.2 420 59.1 38.4 65.0 3 360 39.7 27.8 70.0 370 51.936.0 69.4 400 72.5 44.6 61.5 420 84.2 44.3 52.6 4 360 26.0 18.7 72.1 38053.7 38.2 71.2 400 67.4 44.0 65.3 420 75.3 42.8 56.8 5 360 40.1 28.170.1 380 57.3 39.7 69.3 400 72.9 45.0 61.8 420 85.3 45.2 53.0

From the examples reported above it is clear that the best performance(i. e., the highest yield is obtained for those catalysts (ex. 3 and 4)which fulfill both the requirements of having a carbon content in theprecursor of 1 to 2 wt %, and a residual carbon content afterpre-calcination at 300° C. in air of 0.8 to 1.5 wt %.

What is claimed is:
 1. A process for the preparation of avanadium/phosphorus mixed oxide catalyst precursor comprising reacting asource of vanadium in an organic medium in the presence of a phosphorussource, the organic medium comprising: (a) isobutyl alcohol or a mixtureof isobutyl alcohol and benzyl alcohol, and (b) a polyol in a weightratio (a) to (b) of 99:1 to 5:95.
 2. The process of claim 1, wherein thesource of vanadium is a compound of vanadium or pentavalent vanadium. 3.The process of claim 2, wherein the source of vanadium is vanadiumpentoxide.
 4. The process of claim 2, wherein the phosphorus source isphosphoric acid.
 5. The process of claim 4, wherein the component (a) ofthe organic medium is isobutyl alcohol.
 6. The process of claim 4,wherein the polyol is a C₂₋₆-aliphatic polyol.
 7. The process of claim6, wherein the C₂₋₆aliphatic polyol is a C₂₋₆-alkanediol.
 8. The processof claim 6, wherein the C₂₋₆-aliphatic polyol is a C₂₋₄-alkanediol. 9.The process of claim 8, wherein the reaction takes place at atemperature of 90 to 200° C.
 10. The process of claim 2, wherein thecomponent (a) of the organic medium is isobutyl alcohol.
 11. The processof claim 1, wherein the phosphorus source is phosphoric acid.
 12. Theprocess of claim 1, wherein the component (a) of the organic medium isisobutyl alcohol.
 13. The process of claim 1, wherein the polyol is aC₂₋₆-aliphatic polyol.
 14. The process of claim 13, wherein theC₂₋₆-aliphatic polyol is a C₂₋₆-alkanediol.
 15. The process of claim 14,wherein the C₂₋₆-aliphatic polyol is a C₂₋₆-alkanediol.
 16. The processof claim 1, wherein the reaction takes place at a temperature of 90 to200° C.
 17. A vanadium/phosphorous mixed oxide catalyst precursor havinga carbon content in the range of 0.7 wt. percent to 15 wt. percent, andhaving been prepared by process comprising reacting a source of vanadiumin an organic medium in the presence of a phosphorus source, the organicmedium comprising: (a) isobutyl alcohol or a mixture of isobutyl alcoholand benzyl alcohol, and (b) polyol in a weight ratio (a) to (b) of 99:1to 5:95.
 18. The vanadium/phosphorus mixed oxide catalyst precursor ofclaim 17, wherein, after an additional treatment at 300° C. for 3 hoursin air, the carbon is between 0.7 wt. percent and 3 wt. percent.
 19. Thevanadium/phosphorus mixed oxide catalyst precursor of claim 18, whereinthe carbon content is between 0.8 wt. percent and 1.5 wt. percent. 20.The vanadium/phosphorus mixed oxide catalyst precursor having beenprepared by the process according to claim 16, and having a carboncontent in the range of 0.7 wt. percent to 15 wt. percent.
 21. Thevanadium/phosphorus mixed oxide catalyst precursor of claim 20, wherein,after an additional treatment at 300° C. for 3 hours in air, the carboncontent is between 0.7 wt. percent and 3 wt. percent.
 22. Thevanadium/phosphorus mixed oxide catalyst precursor to claim 21, whereinthe carbon content is between 0.8 wt. percent and 1.5 wt. percent.